1. Field of the Invention
The present invention relates to an image forming apparatus including a developing device having a plurality of developing units and a transfer device that transfers a developed toner image to an image bearing member, and more particularly to an image forming apparatus in which developed toner patches we employed to control toner density.
2. Discussion of the Background
A developing device of an image forming apparatus using electrophotographic image-forming and electrostatic latent image writing employs a two-component developer of a mixture of toner and carrier. Especially in a color image forming apparatus that forms full and multi-color images electrophotographically, the developing device generally employs a two-component developer to produce a color image. In the two-component developer, a toner density control (i.e., a ratio between toner and carrier by weight) is a very important factor to obtain a desired image quality. For example, when toner is used to develop a latent image on an image bearing member such as a photoconductive member, the amount and density of toner decrease in the two-component developer.
In order to maintain a desired image quality, it is necessary precisely to detect the toner density in the developer, to supply toner according to the consumed amount, and to control toner density at a certain value.
In a background method of measuring toner density in a developer, a latent image of toner patch having a generated toner pattern is formed, for example, in a 20 mm by 20 mm square, during each image forming process on an image bearing member (i.e., a photoconductive member) in a non-image area on the image bearing member, that is, in an area between image forming areas. The latent image of the toner patch is developed by toner, and then the amount of toner on the toner patch is measured by a reflection type optical sensor. The toner density is controlled by maintaining a standard value of toner density.
The above-described toner density measuring method has been increasingly employed in a full color copier, too. The full color copier has a plurality of developing units, and the density of toner in each color developing unit is required to be kept adequately for a desired image quality. In a color image forming process, each toner image developed by each color toner on a first image bearing member (i.e., a photoconductive member) is transferred to a second image bearing member (i.e., a transfer belt) each time color image forming is performed and is superimposed on the surface thereof with the leading edge of each color toner image aligned. The above-described toner patches developed by each color toner in a non-image area on the first image bearing member are also transferred to the second image bearing member and superimposed on the surface thereof.
Immediately after transferring a toner image and a toner patch to the second image bearing member, a residual toner on the first image bearing member is cleaned by a first cleaning device for image forming and developing of a next color image. On the second image bearing member, full color toner images and toner patches are superimposed thereon, and only full color toner images are transferred to a transfer sheet. Superimposed toner patches remain on the second bearing member. Then, a second cleaning device including a cleaning blade cleans residual toner and the superimposed toner patches on the second image bearing member. Because the cleaning blade needs to scrape the superimposed toner patches strongly, the load on the second cleaning device increases. As a result, a vibration of the cleaning blade occurs, and the residual toner and toner patches remain on the second image bearing member without being adequately cleaned. Toner remaining on the second image bearing member stains the next color toner image transferred from the first image bearing member.
Japanese Laid-open Patent Publication No. 10-149009 describes an image forming apparatus in which toner patches for measuring and controlling the density of toner are formed in a line along the longitudinal direction of a photoconductive drum. In this image forming apparatus, a plurality of photo sensors of a toner density detecting device detect each toner patch. However, such a mechanism for the toner density detecting device increases the cost of the image forming apparatus.